Incorporation of hydrophilic functional agents within a hydrophobic matrix has been a conventional challenge in materials science. This challenge can be addressed by forming stable liquid emulsions and using them as templates to form solid-like composites. Emulsions are the mixture of immiscible liquids and spherical dispersed droplets are within a continuous phase. The liquid emulsion droplets need to be robust and have long-term stability, so that the structure of the emulsion droplets throughout the drying process should be maintained. Although micro-emulsions are thermodynamically stable and require little attention for a stabilizing system, emulsion droplets tend to coalesce and phase separate before the templating process, unless caution is taken. It is well established that, as an alternative to surface active molecules, small solid particles attach at fluid/fluid interfaces of two immiscible mediums when the particles are partially wettable by both mediums. This mechanism for use in emulsion templates due to their strong stability is known as “Pickering emulsions”.
In a study, Imhof and Pine have templated micro-emulsions of droplet size ranging from 100 to 200 nm (Imhof A, Pine D J (1997) Ordered macroporous materials by emulsion templating. Nature 389: 948-951). They have employed sol-gel processing to cure the continuous phase. The results showed that they were able to make materials with a well: defined pore size difference of, 20%. They have obtained a porous monolith by removal of oil droplets within aqueous continuous medium and gelation of the aqueous medium by sol-gel method (Imhof A, Pine D J (1998) Uniform macroporous ceramics and plastics by emulsion templating. Chemical Engineering & Technology 21: 682-685). In another study, Binks prepared a porous silica monolith by silica particles alone (Binks B P (2002) Macroporous silica from solid-stabilized emulsion templates. Advanced Materials 14: 1824-1827). Such porous monoliths are also proposed to be used as adsorbents, catalytic supports, light-weight structural materials, insulators besides their potential application as filters.
The importance of self-assembly of target materials at the interface to form stable capsules was investigated in a study of Wei and Wan. The authors used polyaniline coated hollow microspheres for self-assembly of aniline monomers around oil-in-water (o/w) droplets in emulsion, and polymerized the shell subsequently. Such capsules are promising to be used as delivery vehicles for controlled-release encapsulation, drug delivery, protection of biologically active agents.
Macroporous systems are proposed to serve mostly as filtering systems, whereas the capsules have been considered to encapsulate either nanoparticles or biomaterials such as drugs, food and cosmetics.